What are 'true breeding lines' that are used to study inheritance pattern of traits in plants ?

True breeding occurs when plants produce only offspring of the same variety when they self-pollinate. For this to occur the parents are homozygous for a trait which means the parents must be both dominant or both recessive. For ex. A plant with that has red flowers will produce only seeds that grow into plants that have red flowers. In true breeding the trait is passed on to all subsequent generations.